Our present understanding of segmental motor control has been greatly influenced by investigations of the lumbosacral spinal cord and hindlimb muscles of the cat. However, to focus largely on this region of the spinal cord and the muscles it ultimately controls, certain fundamental and unique features of segmental motor control might go unnoticed. For a fuller understanding of the segmental motor control system, it is the intent of this project to investigate this system in the cat forelimb. The specific aims of this project are directed towards increasing our knowledge of: (1) the synaptic organization of distal forelimb motor units; and (2) the principles which govern the formation of specific afferent connections within the spinal cord by investigating the central effects of individual distal forelimb muscle spindle afferent fibers. This will be accomplished by applying a combination of neuroanatomical and electrophysiological techniques. Our long-range objective is to conduct a thorough investigation of the cervical spinal cord and muscles of the forelimb of the cat. Hopefully, this effort will contribute to our understanding of how forelimb motoneurons are controlled segmentally, what is the nature of their synaptic input, what organizational principles are common between the forelimb and other areas of spinal motor organization, and what specializations are unique to the forelimb because of the motor tasks it must perform. Without a clear understanding of the basic properties and design features of the segmental motor control system at all levels of the neuraxis, we will not be able to fully understand how this system is affected under a variety of neurological disorders. Moreover, information derived from the proposed studies will ultimately have practical implications for many orthopedic procedures, such as muscle transposition, surgical reinnervation, and implantation of limb prosthetic devices.